Current standards for attaching implants such as plates to bones often constitute screws in a plurality of shapes. The purpose of these screws is to transfer the load from one bone fragment to the plate or nail and back to a secondary bone fragment.
To ensure this load transfer, the screws must have a good connection in the bone and the plate. The connection between the plate and the screw can be achieved with an angular stable system, which transfers the load through a form-fitted connection, not through friction between the plate and screw. Recent advancements in locked plating systems have enabled the clinician to easily achieve this angular stable construct in normal healthy bone.
However, even angular stable constructs fail in osteoporotic bone because of a lack of stability in the bone to screw interface. Osteoporosis is characterized by a reduction of bone mass and also by alteration in the architecture of the bone. The trabecular bone structure is changing significantly. These changes are taking place after the 6th decade of life and women are more affected than men. The whole skeleton is affected by osteoporosis, with varying amount of impact throughout the body. The regions most severely affected by osteoporosis are the spine, proximal femur, distal radius, proximal humerus and proximal tibia. Treating osteoporotic fractures in these areas can be very challenging for the surgeon because the screws can not find sufficient purchase in the weak trabecular structure. There is at times a complete absence of bone where the surgeon would normally place the screw, such as in the proximal humerus.
One method of improving the fixation between the screw and the osteoporotic bone has been to augment the bone with a hardenable biomaterial such as PMMA cement or calcium phosphate (CaP) cement. Each of these methods has disadvantages.
For example, disadvantages of using PMMA include the permanent non-resorbing nature of PMMA. It remains within the body after the fracture has healed and removal of the material is nearly impossible once implanted. The stiffness of PMMA is in excess of the surrounding bone creating excess stress at the interface to the bone. PMMA can release monomer during the curing process and the monomer can become vascularized. PMMA releases noxious fumes during the mixing and curing process requiring special ventilation. PMMA is initially too runny to handle and can quickly become too difficult to implant and the state of the material is not reversible. PMMA has a minimal ductility, can bind to metals making screw removal difficult, and can be difficult to control the direction of implantation. PMMA can either extravasate into the canal of the diaphysis (rendering it ineffective), into the joint space or become vascularized (leading to an embolism). Furthermore, PMMA includes a risk of thermal necrosis due to the exothermic reaction during curing.
There has been recent interest in using calcium phosphate cements for augmentation of screws and other fracture fixation devices. Calcium phosphate will slowly remodel over time and does not contain a monomer, however, it has the following deficiencies. The stiffness of calcium phosphate is in excess of the surrounding bone creating excess stress at the interface to the bone. Calcium phosphate will inherently not perfuse into surrounding bone without the addition of a flow enhancing agent. Calcium phosphates are subject to phase separation if they are overpressurized. Calcium phosphate includes a variable time and temperature dependent rheology. The calcium phosphate materials will not properly set unless the surrounding tissue is near 37° C. It can be difficult to control the direction of implantation of calcium phosphates and calcium phosphates can either extravasate into the canal of the diaphysis (rendering it ineffective), into the joint space or become vascularized (leading to an embolism). Calcium phosphates include suboptimal mechanical properties while they often have adequate compressive strength, they have little tensile strength, flexural strength or ductility. Furthermore, additional calcium phosphate material will not bond to calcium phosphate material that has already set and the drillability and screwability of the calcium phosphate is limited.
Improvements have been made to calcium phosphate cements such as adding reinforcing fibers and flow enhancing agents. The addition of reinforcing fibers actually renders the cement less advantageous for hardware augmentation since the fiber will be filtered by the trabecular structure surrounding the hardware and will impede perfusion. Flow enhancing agents such as hyaluronic acid will improve some of the handling properties of the calcium phosphate cements and will allow perfusion. However the material remains suboptimal for the application.
There also exists a need for improved materials when no fixation hardware is used. Such instances would include augmenting osteoporotic bone such as a vertebral body or to fill voids where the bone has been compressed due to trauma. The purpose of the material in this case is not to fixate the hardware to the bone, but rather to directly replace or augment the bone. PMMA is commonly used for vertebroplasty procedures but suffers from many of the problems stated above. Calcium phosphate cements can be used as well, but also with the above limitations. Further, in these types of applications there often exists a need to reduce the fractures or to compress the surrounding bone, which the existing materials are not capable of doing.